Network Working Group K. Morneault
Request for Comments: 3057 Cisco Systems
Category: Standards Track S. Rengasami
M. Kalla
Telcordia Technologies
G. Sidebottom
Nortel Networks
February 2001
ISDN Q.921-User Adaptation Layer
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document defines a protocol for backhauling of ISDN Q.921 User
messages over IP using the Stream Control Transmission Protocol
(SCTP). This protocol would be used between a Signaling Gateway (SG)
and Media Gateway Controller (MGC). It is assumed that the SG
receives ISDN signaling over a standard ISDN interface.
Morneault, et al. Standards Track [Page 1]

RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
Signaling Transport [4]. The delivery mechanism SHOULD meet the
following criteria:
* Support for transport of the Q.921 / Q.931 boundary primitives
* Support for communication between Layer Management modules on SG
and MGC
* Support for management of active associations between SG and MGC
This document supports both ISDN Primary Rate Access (PRA) as well as
Basic Rate Access (BRA) including the support for both point-to-point
and point-to-multipoint modes of communication. This support
includes Facility Associated Signaling (FAS), Non-Facility Associated
Signaling (NFAS) and NFAS with backup D channel. QSIG adaptation
layer requirements do not differ from Q.931 adaptation layer, hence;
the procedures described in this document are also applicable for a
QSIG adaptation layer. For simplicity, only Q.931 will be mentioned
in the rest of this document.
1.2 Terminology
Interface - For the purposes of this document an interface supports
the relevant ISDN signaling channel. This signaling channel MAY be a
16 kbps D channel for an ISDN BRA as well as 64 kbps primary or
backup D channel for an ISDN PRA. For QSIG, the signaling channel is
a Qc channel.
Q.921-User - Any protocol normally using the services of the ISDN
Q.921 (e.g., Q.931, QSIG, etc.).
Backhaul - A SG terminates the lower layers of an SCN protocol and
backhauls the upper layer(s) to MGC for call processing. For the
purposes of this document the SG terminates Q.921 and backhauls Q.931
to MGC.
Association - An association refers to a SCTP association. The
association will provide the transport for the delivery of Q.921-User
protocol data units and IUA adaptation layer peer messages.
Stream - A stream refers to an SCTP stream; a uni-directional logical
channel established from one SCTP endpoint to another associated SCTP
endpoint, within which all user messages are delivered in-sequence
except for those submitted to the un-ordered delivery service.
Interface Identifier - The Interface Identifier identifies the
physical interface at the SG for which the signaling messages are
sent/received. The format of the Interface Identifier parameter can
be text or integer, the values of which are assigned according to
Morneault, et al. Standards Track [Page 3]

RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
network operator policy. The values used are of local significance
only, coordinated between the SG and ASP. Significance is not
implied across SGs served by an AS.
Application Server (AS) - A logical entity serving a specific
application instance. An example of an Application Server is a MGC
handling the Q.931 and call processing for D channels terminated by
the Signaling Gateways. Practically speaking, an AS is modeled at
the SG as an ordered list of one or more related Application Server
Processes (e.g., primary, secondary, tertiary).
Application Server Process (ASP) - A process instance of an
Application Server. Examples of Application Server Processes are
primary or backup MGC instances.
Fail-over - The capability to re-route signaling traffic as required
between related ASPs in the event of failure or unavailability of the
currently used ASP (e.g., from primary MGC to back-up MGC). Fail-
over also applies upon the return to service of a previously
unavailable process.
Layer Management - Layer Management is a nodal function that handles
the inputs and outputs between the IUA layer and a local management
entity.
Network Byte Order - Most significant byte first, a.k.a Big Endian.
Host - The computing platform that the ASP process is running on.
1.3 IUA Overview
The architecture that has been defined [4] for SCN signaling
transport over IP uses multiple components, including an IP transport
protocol, a signaling common transport protocol and an adaptation
module to support the services expected by a particular SCN signaling
protocol from its underlying protocol layer.
This document defines an adaptation module that is suitable for the
transport of ISDN Q.921-User (e.g., Q.931) messages.
1.3.1 Example - SG to MGC
In a Signaling Gateway, it is expected that the ISDN signaling is
received over a standard ISDN network termination. The SG then
provides interworking of transport functions with IP Signaling
Transport, in order to transport the Q.931 signaling messages to the
MGC where the peer Q.931 protocol layer exists, as shown below:
Morneault, et al. Standards Track [Page 4]

RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
****** ISDN ****** IP *******
* EP *---------------* SG *--------------* MGC *
****** ****** *******
+-----+ +-----+
|Q.931| (NIF) |Q.931|
+-----+ +----------+ +-----+
| | | | IUA| | IUA |
| | | +----+ +-----+
|Q.921| |Q.921|SCTP| |SCTP |
| | | +----+ +-----+
| | | | IP | | IP |
+-----+ +-----+----+ +-----+
NIF - Nodal Interworking Function
EP - ISDN End Point
SCTP - Stream Control Transmission Protocol (Refer to [3])
IUA - ISDN User Adaptation Layer Protocol
It is recommended that the IUA use the services of the Stream Control
Transmission Protocol (SCTP) as the underlying reliable common
signaling transport protocol. The use of SCTP provides the following
features:
- explicit packet-oriented delivery (not stream-oriented)
- sequenced delivery of user messages within multiple streams,
with an option for order-of-arrival delivery of individual user
messages,
- optional multiplexing of user messages into SCTP datagrams,
- network-level fault tolerance through support of multi-homing
at either or both ends of an association,
- resistance to flooding and masquerade attacks, and
- data segmentation to conform to discovered path MTU size
There are scenarios without redundancy requirements and scenarios in
which redundancy is supported below the transport layer. In these
cases, the SCTP functions above MAY NOT be a requirement and TCP can
be used as the underlying common transport protocol.
1.3.2 Support for the management of SCTP associations between the SG and ASPs
The IUA layer at the SG maintains the availability state of all
dynamically registered remote ASPs, in order to manage the SCTP
Associations and the traffic between the SG and ASPs. As well, the
active/inactive state of remote ASP(s) are also maintained. Active
ASPs are those currently receiving traffic from the SG.
Morneault, et al. Standards Track [Page 5]

RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
The IUA layer MAY be instructed by local management to establish an
SCTP association to a peer IUA node. This can be achieved using the
M-SCTP ESTABLISH primitive to request, indicate and confirm the
establishment of an SCTP association with a peer IUA node.
The IUA layer MAY also need to inform local management of the status
of the underlying SCTP associations using the M-SCTP STATUS request
and indication primitive. For example, the IUA MAY inform local
management of the reason for the release of an SCTP association,
determined either locally within the IUA layer or by a primitive from
the SCTP.
1.3.3 Signaling Network Architecture
A Signaling Gateway is used to support the transport of Q.921-User
signaling traffic to one or more distributed ASPs (e.g., MGCs).
Clearly, the IUA protocol is not designed to meet the performance and
reliability requirements for such transport by itself. However, the
conjunction of distributed architecture and redundant networks does
allow for a sufficiently reliable transport of signaling traffic over
IP. The IUA protocol is flexible enough to allow its operation and
management in a variety of physical configurations, enabling Network
Operators to meet their performance and reliability requirements.
To meet the ISDN signaling reliability and performance requirements
for carrier grade networks, Network Operators SHOULD ensure that
there is no single point of failure provisioned in the end-to-end
network architecture between an ISDN node and an IP ASP.
Depending of course on the reliability of the SG and ASP functional
elements, this can typically be met by the provision of redundant
QOS-bounded IP network paths for SCTP Associations between SCTP End
Points, and redundant Hosts, and redundant SGs. The distribution of
ASPs within the available Hosts is also important. For a particular
Application Server, the related ASPs SHOULD be distributed over at
least two Hosts.
An example logical network architecture relevant to carrier-grade
operation in the IP network domain is shown in Figure 1 below:
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
Host1
******** **************
* *_________________________________________* ******** *
* * _________* * ASP1 * *
* SG1 * SCTP Associations | * ******** *
* *_______________________ | * *
******** | | **************
| |
******** | |
* *_______________________________|
* * |
* SG2 * SCTP Associations |
* *____________ |
* * | | Host2
******** | | **************
| |_________________* ******** *
|____________________________* * ASP1 * *
* ******** *
* *
**************
.
.
.
Figure 2 - Logical Model Example
For carrier grade networks, the failure or isolation of a particular
ASP SHOULD NOT cause stable calls to be dropped. This implies that
ASPs need, in some cases, to share the call state or be able to pass
the call state between each other. However, this sharing or
communication of call state information is outside the scope of this
document.
1.3.4 ASP Fail-over Model and Terminology
The IUA layer supports ASP fail-over functions in order to support a
high availability of call processing capability. All Q.921-User
messages incoming to an SG are assigned to a unique Application
Server, based on the Interface Identifier of the message.
The Application Server is, in practical terms, a list of all ASPs
configured to process Q.921-User messages from certain Interface
Identifiers. One or more ASPs in the list are normally active (i.e.,
handling traffic) while any others MAY be unavailable or inactive, to
be possibly used in the event of failure or unavailability of the
active ASP(s).
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
The fail-over model supports an n+k redundancy model, where n ASP(s)
are the minimum number of redundant ASPs required to handle traffic
and k ASPs are available to take over for a failed or unavailable
ASP. Note that 1+1 active/standby redundancy is a subset of this
model. A simplex 1+0 model is also supported as a subset, with no
ASP redundancy.
To avoid a single point of failure, it is recommended that a minimum
of two ASPs be in the list, resident in separate hosts and therefore
available over different SCTP Associations. For example, in the
network shown in Figure 2, all messages from a particular D Channel
(Interface Identifier) could be sent to ASP1 in Host1 or ASP1 in
Host2. The AS list at SG1 might look like the following:
Interface Identifier(s) - Application Server #1
ASP1/Host1 - State=Up, Active
ASP1/Host2 - State=Up, Inactive
In this 1+1 redundancy case, ASP1 in Host1 would be sent any incoming
message for the Interface Identifiers registered. ASP1 in Host2
would normally be brought to the active state upon failure of, or
loss of connectivity to, ASP1/Host1. In this example, both ASPs are
Up, meaning that the related SCTP association and far-end IUA peer is
ready.
The AS List at SG1 might also be set up in load-share mode as shown
below:
Interface Identifier(s) - Application Server #1
ASP1/Host1 - State=Up, Active
ASP1/Host2 - State=Up, Active
In this case, both the ASPs would be sent a portion of the traffic.
In the process of fail-over, it is recommended that in the case of
ASPs supporting call processing, stable calls do not get released.
It is possible that calls in transition MAY fail, although measures
of communication between the ASPs involved can be used to mitigate
this problem. For example, the two ASPs MAY share call state via
shared memory, or MAY use an ASP to ASP protocol to pass call state
information. The ASP to ASP protocol is outside the scope of this
document.
1.3.5 Client/Server Model
It is recommended that the SG and ASP be able to support both client
and server operation. The peer endpoints using IUA SHOULD be
configured so that one always takes on the role of client and the
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
other the role of server for initiating SCTP associations. The
default orientation would be for the SG to take on the role of server
while the ASP is the client. In this case, ASPs SHOULD initiate the
SCTP association to the SG.
The SCTP (and UDP/TCP) Registered User Port Number Assignment for IUA
is 9900.
1.4 Services Provided by the IUA Layer1.4.1 Support for transport of Q.921/Q.931 boundary primitives
In the backhaul scenario, the Q.921/Q.931 boundary primitives are
exposed. IUA layer needs to support all of the primitives of this
boundary to successfully backhaul Q.931.
This includes the following primitives [1]:
DL-ESTABLISH
The DL-ESTABLISH primitives are used to request, indicate and confirm
the outcome of the procedures for establishing multiple frame
operation.
DL-RELEASE
DL-RELEASE primitives are used to request, indicate, and confirm the
outcome of the procedures for terminating a previously established
multiple frame operation, or for reporting an unsuccessful
establishment attempt.
DL-DATA
The DL-DATA primitives are used to request and indicate layer 3
(Q.931) messages which are to be transmitted, or have been received,
by the Q.921 layer using the acknowledged information transfer
service.
DL-UNIT DATA
The DL-UNIT DATA primitives are used to request and indicate layer 3
(Q.931) messages which are to be transmitted, by the Q.921 layer
using the unacknowledged information transfer service.
Morneault, et al. Standards Track [Page 9]

RFC 3057 ISDN Q.921-User Adaptation Layer February 20011.4.2 Support for communication between Layer Management modules on SG and MGC
It is envisioned that the IUA layer needs to provide some services
that will facilitate communication between Layer Management modules
on the SG and MGC. These primitives are pointed out in [2], which
are shown below:
M-TEI STATUS
The M-TEI STATUS primitives are used to request, confirm and indicate
the status (assigned/unassigned) of a TEI.
M-ERROR
The M-ERROR primitive is used to indicate an error with a received
IUA message (e.g., interface identifier value is not known to the
SG).
1.4.3 Support for management of active associations between SG and MGC
A set of primitives between the IUA layer and the Layer Management
are defined below to help the Layer Management manage the SCTP
association(s) between the SG and MGC. The IUA layer can be
instructed by the Layer Management to establish an SCTP association
to a peer IUA node. This procedure can be achieved using the M-SCTP
ESTABLISH primitive.
M-SCTP ESTABLISH
The M-SCTP ESTABLISH primitives are used to request, indicate, and
confirm the establishment of an SCTP association to a peer IUA node.
M-SCTP RELEASE
The M-SCTP RELEASE primitives are used to request, indicate, and
confirm the release of an SCTP association to a peer IUA node.
The IUA layer MAY also need to inform the status of the SCTP
associations to the Layer Management. This can be achieved using the
M-SCTP STATUS primitive.
M-SCTP STATUS
The M-SCTP STATUS primitives are used to request and indicate the
status of the underlying SCTP association(s).
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
The Layer Management MAY need to inform the IUA layer of an AS/ASP
status (i.e., failure, active, etc.), so that messages can be
exchanged between IUA layer peers to stop traffic to the local IUA
user. This can be achieved using the M-ASP STATUS primitive.
M-ASP STATUS
The ASP status is stored inside IUA layer on both the SG and MGC
sides. The M-ASP STATUS primitive can be used by Layer Management to
request the status of the Application Server Process from the IUA
layer. This primitive can also be used to indicate the status of the
Application Server Process.
M-ASP-UP
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Up message for the Application Server Process. It can also be used
to generate an ASP Up Acknowledgement.
M-ASP-DOWN
The M-ASP-DOWN primitive can be used by Layer Management to send a
ASP Down message for the Application Server Process. It can also be
used to generate an ASP Down Acknowledgement.
M-ASP-ACTIVE
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Active message for the Application Server Process. It can also be
used to generate an ASP Active Acknowledgement.
M-ASP-INACTIVE
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Inactive message for the Application Server Process. It can also be
used to generate an ASP Inactive Acknowledgement.
M-AS STATUS
The M-AS STATUS primitive can be used by Layer Management to request
the status of the Application Server. This primitive can also be
used to indicate the status of the Application Server.
Morneault, et al. Standards Track [Page 11]

RFC 3057 ISDN Q.921-User Adaptation Layer February 20011.5 Functions Implemented by the IUA Layer1.5.1 Mapping
The IUA layer MUST maintain a map of the Interface Identifier to a
physical interface on the Signaling Gateway. A physical interface
would be a T1 line, E1 line, etc., and could include the TDM
timeslot. In addition, for a given interface the SG MUST be able to
identify the associated signaling channel. IUA layers on both SG and
MGC MAY maintain the status of TEIs and SAPIs.
The SG maps an Interface Identifier to an SCTP association/stream
only when an ASP sends an ASP Active message for a particular
Interface Identifier. It MUST be noted, however, that this mapping
is dynamic and could change at any time due to a change of ASP state.
This mapping could even temporarily be invalid, for example during
failover of one ASP to another. Therefore, the SG MUST maintain the
states of AS/ASP and reference them during the routing of an messages
to an AS/ASP.
One example of the logical view of relationship between D channel,
Interface Identifier, AS and ASP in the SG is shown below:
/---------------------------------------------------+
/ /------------------------------------------------|--+
/ / v |
/ / +----+ act+-----+ +-------+ -+--+-|+--+-
D chan1-------->|IID |-+ +-->| ASP |--->| Assoc | v
/ +----+ | +----+ | +-----+ +-------+ -+--+--+--+-
/ +->| AS |--+ Streams
/ +----+ | +----+ stb+-----+
D chan2-------->|IID |-+ | ASP |
+----+ +-----+
where IID = Interface Identifier
Note that an ASP can be in more than one AS.
1.5.2 Status of ASPs
The IUA layer on the SG MUST maintain the state of the ASPs it is
supporting. The state of an ASP changes because of reception of
peer-to-peer messages (ASPM messages as described in Section 3.3.2)
or reception of indications from the local SCTP association. ASP
state transition procedures are described in Section 4.3.1.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
At a SG, an Application Server list MAY contain active and inactive
ASPs to support ASP load-sharing and fail-over procedures. When, for
example, both a primary and a back-up ASP are available, IUA peer
protocol is required to control which ASP is currently active. The
ordered list of ASPs within a logical Application Server is kept
updated in the SG to reflect the active Application Server
Process(es).
Also the IUA layer MAY need to inform the local management of the
change in status of an ASP or AS. This can be achieved using the M-
ASP STATUS or M-AS STATUS primitives.
1.5.3 SCTP Stream Management
SCTP allows a user specified number of streams to be opened during
the initialization. It is the responsibility of the IUA layer to
ensure proper management of these streams. Because of the
unidirectional nature of streams, an IUA layer is not aware of the
stream number to Interface Identifier mapping of its peer IUA layer.
Instead, the Interface Identifier is in the IUA message header.
The use of SCTP streams within IUA is recommended in order to
minimize transmission and buffering delay, therefore improving the
overall performance and reliability of the signaling elements. It is
recommended that a separate SCTP stream is used for each D channel.
1.5.4 Seamless Network Management Interworking
The IUA layer on the SG SHOULD pass an indication of unavailability
of the IUA-User (Q.931) to the local Layer Management, if the
currently active ASP moves from the ACTIVE state. The Layer
Management could instruct Q.921 to take some action, if it deems
appropriate.
Likewise, if an SCTP association fails, the IUA layer on both the SG
and ASP sides MAY generate Release primitives to take the data links
out-of-service.
1.5.5 Congestion Management
If the IUA layer becomes congested (implementation dependent), it MAY
stop reading from the SCTP association to flow control from the peer
IUA.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
Parameter Length: 16 bits (unsigned integer)
The Parameter Length field contains the size of the parameter in
bytes, including the Parameter Tag, Parameter Length, and Parameter
Value fields. The Parameter Length does not include any padding
bytes.
Parameter Value: variable-length
The Parameter Value field contains the actual information to be
transferred in the parameter.
The total length of a parameter (including Tag, Parameter Length and
Value fields) MUST be a multiple of 4 bytes. If the length of the
parameter is not a multiple of 4 bytes, the sender pads the Parameter
at the end (i.e., after the Parameter Value field) with all zero
bytes. The length of the padding is NOT included in the parameter
length field. A sender SHOULD NEVER pad with more than 3 bytes. The
receiver MUST ignore the padding bytes.
3.2 IUA Message Header
In addition to the common message header, there will be a specific
message header for QPTM and the TEI Status MGMT messages. The IUA
message header will immediately follow the Common header in these
messages.
This message header will contain the Interface Identifier and Data
Link Connection Identifier (DLCI). The Interface Identifier
identifies the physical interface terminating the signaling channel
at the SG for which the signaling messages are sent/received. The
format of the Interface Identifier parameter can be text or integer.
The Interface Identifiers are assigned according to network operator
policy. The integer values used are of local significance only,
coordinated between the SG and ASP.
The integer formatted Interface Identifier MUST be supported. The
text formatted Interface Identifier MAY optionally be supported.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
SAPI: Service Access Point Identifier, 3rd through 8th bits in octet
1 (6 bits)
TEI: Terminal Endpoint Identifier, 2nd through 8th bits in octet 2
(7 bits)
The DLCI field (including the SAPI and TEI) is coded in accordance
with Q.921.
3.3 IUA Messages
The following section defines the messages and parameter contents.
The IUA messages will use the common message header (Figure 3) and
the IUA message header (Figure 4 and Figure 5).
3.3.1 Q.921/Q.931 Boundary Primitives Transport (QPTM) Messages3.3.1.1 Establish Messages (Request, Confirm, Indication)
The Establish Messages are used to establish a data link on the
signaling channel or to confirm that a data link on the signaling
channel has been established. The MGC controls the state of the D
channel. When the MGC desires the D channel to be in-service, it
will send the Establish Request message.
When the MGC sends an IUA Establish Request message, the MGC MAY
start a timer. This timer would be stopped upon receipt of an IUA
Establish Confirm or Establish Indication. If the timer expires, the
MGC would re-send the IUA Establish Request message and restart the
timer. In other words, the MGC MAY continue to request the
establishment of the data link on periodic basis until the desired
state is achieved or take some other action (notify the Management
Layer).
When the SG receives an IUA Establish Request from the MGC, the SG
shall send the Q.921 Establish Request primitive to the its Q.921
entity. In addition, the SG shall map any response received from the
Q.921 entity to the appropriate message to the MGC. For example, if
the Q.921 entity responds with a Q.921 Establish Confirm primitive,
the IUA layer shall map this to an IUA Establish Confirm message. As
another example, if the IUA Layer receives a Q.921 Release Confirm or
Release Indication as an apparent response to the Q.921 Establish
Request primitive, the IUA Layer shall map these to the corresponding
IUA Release Confirm or Release Indication messages.
The Establish messages contain the common message header followed by
IUA message header. It does not contain any additional parameters.
Morneault, et al. Standards Track [Page 22]

RFC 3057 ISDN Q.921-User Adaptation Layer February 20013.3.1.2 Release Messages (Request, Indication, Confirmation)
The Release Request message is used to release the data link on the
signaling channel. The Release Confirm and Indication messages are
used to indicate that the data link on the signaling channel has been
released.
If a response to the Release Request message is not received, the MGC
MAY resend the Release Request message. If no response is received,
the MGC can consider the data link as being released. In this case,
signaling traffic on that D channel is not expected from the SG and
signaling traffic will not be sent to the SG for that D channel.
The Release messages contain the common message header followed by
IUA message header. The Release confirm message is in response to a
Release Request message and it does not contain any additional
parameters. The Release Request and Indication messages contain the
following parameter:
REASON
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0xf) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The valid values for Reason are shown in the following table.
Define Value Description
RELEASE_MGMT 0x0 Management layer generated release.
RELEASE_PHYS 0x1 Physical layer alarm generated release.
RELEASE_DM 0x2 Specific to a request. Indicates Layer 2
SHOULD release and deny all requests from
far end to establish a data link on the
signaling channel (i.e., if SABME is
received send a DM)
RELEASE_OTHER 0x3 Other reasons
Note: Only RELEASE_MGMT, RELEASE_DM and RELEASE_OTHER are valid
reason codes for a Release Request message.
3.3.1.3 Data Messages (Request, Indication)
The Data message contains an ISDN Q.921-User Protocol Data Unit (PDU)
corresponding to acknowledged information transfer service.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
Interface Identifiers are being used, the ASP can also send ranges of
Interface Identifiers (Type 0x8). Interface Identifier types Integer
(0x1) and Integer Range (0x8) are allowed in the same message. Text
formatted Interface Identifiers (0x3) cannot be used with either
Integer (0x1) or Integer Range (0x8) types.
If no Interface Identifiers are included, the message is for all
provisioned Interface Identifiers within the AS(s) in which the ASP
is provisioned. If only a subset of Interface Identifiers are
included, the ASP is noted as Active for all the Interface
Identifiers provisioned for that AS.
Note: If the optional Interface Identifier parameter is present, the
integer formatted Interface Identifier MUST be supported, while the
text formatted Interface Identifier MAY be supported.
The format and description of the optional Info String parameter is
the same as for the ASP Up message (See Section 3.3.2.1.).
An SG that receives an ASPAC with an incorrect Traffic Mode Type for
a particular Interface Identifier will respond with an Error Message
(Cause: Unsupported Traffic Handling Mode).
3.3.2.6 ASP Active Ack
The ASPAC Ack message is used to acknowledge an ASP-Active message
received from a remote IUA peer.
The ASPAC Ack message contains the following parameters:
Traffic Mode Type (Mandatory)
Interface Identifier (Optional)
- Combination of integer and integer ranges, OR
- string (text formatted)
INFO String (Optional)
Morneault, et al. Standards Track [Page 30]

RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
The format for the BEAT message is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag = 9 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
| Heartbeat Data * |
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Heartbeat Data parameter contents are defined by the sending
node. The Heartbeat Data could include, for example, a Heartbeat
Sequence Number and, or Timestamp. The receiver of a Heartbeat
message does not process this field as it is only of significance to
the sender. The receiver MUST respond with a Heartbeat Ack message.
3.3.2.10 Heartbeat Ack (BEAT-Ack)
The Heartbeat Ack message is sent in response to a received Heartbeat
message. It includes all the parameters of the received Heartbeat
message, without any change.
3.3.3 Layer Management (MGMT) Messages3.3.3.1 Error (ERR)
The Error message is used to notify a peer of an error event
associated with an incoming message. For example, the message type
might be unexpected given the current state, or a parameter value
might be invalid.
The Error message will only have the common message header. The
Error message contains the following parameters:
Error Code
Diagnostic Information (optional)
Morneault, et al. Standards Track [Page 38]

RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0xc) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x7) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Diagnostic Information* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Code parameter indicates the reason for the Error Message.
The Error parameter value can be one of the following values:
Invalid Version 0x01
Invalid Interface Identifier 0x02
Unsupported Message Class 0x03
Unsupported Message Type 0x04
Unsupported Traffic Handling Mode 0x05
Unexpected Message 0x06
Protocol Error 0x07
Unsupported Interface Identifier Type 0x08
Invalid Stream Identifier 0x09
Unassigned TEI 0x0a
Unrecognized SAPI 0x0b
Invalid TEI, SAPI combination 0x0c
The "Invalid Version" error would be sent if a message was received
with an invalid or unsupported version. The Error message would
contain the supported version in the Common header. The Error
message could optionally provide the supported version in the
Diagnostic Information area.
The "Invalid Interface Identifier" error would be sent by a SG if an
ASP sends a message with an invalid (unconfigured) Interface
Identifier value.
The "Unsupported Traffic Handling Mode" error would be sent by a SG
if an ASP sends an ASP Active with an unsupported Traffic Handling
Mode. An example would be a case in which the SG did not support
load-sharing.
The "Unexpected Message" error would be sent by an ASP if it received
a QPTM message from an SG while it was in the Inactive state (the ASP
could optionally drop the message and not send an Error). It would
Morneault, et al. Standards Track [Page 39]

RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
also be sent by an ASP if it received a defined and recognized
message that the SG is not expected to send (e.g., if the MGC
receives an IUA Establish Request message).
The "Protocol Error" error would be sent for any protocol anomaly
(i.e., a bogus message).
The "Invalid Stream Identifier" error would be sent if a message was
received on an unexpected SCTP stream (i.e., a MGMT message was
received on a stream other than "0").
The "Unsupported Interface Identifier Type" error would be sent by a
SG if an ASP sends a Text formatted Interface Identifier and the SG
only supports Integer formatted Interface Identifiers. When the ASP
receives this error, it will need to resend its message with an
Integer formatted Interface Identifier.
The "Unsupported Message Type" error would be sent if a message with
an unexpected or unsupported Message Type is received.
The "Unsupported Message Class" error would be sent if a message with
an unexpected or unsupported Message Class is received.
The "Unassigned TEI" error may be used when the SG receives an IUA
message that includes a TEI which has not been assigned or recognized
for use on the indicated ISDN D-channel.
The "Unrecognized SAPI" error would handle the case of using a SAPI
that is not recognized by the SG. The "Invalid TEI, SAPI
combination" error identify errors where the TEI is assigned and the
the SAPI is recognized, but the combination is not valid for the
interface (e.g., on a BRI the MGC tries to send Q.921 Management
messages via IUA when Layer Management at the SG SHOULD be performing
this function).
The optional Diagnostic information can be any information germane to
the error condition, to assist in identification of the error
condition. To enhance debugging, the Diagnostic information could
contain the first 40 bytes of the offending message.
3.3.3.2 Notify (NTFY)
The Notify message used to provide an autonomous indication of IUA
events to an IUA peer.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
These notifications are sent from an SG to an ASP upon a change in
status of a particular Application Server. The value reflects the
new state of the Application Server.
If the Status Type is Other, then the following Status Information
values are defined:
Value Description
1 Insufficient ASP resources active in AS
2 Alternate ASP Active
These notifications are not based on the SG reporting the state
change of an ASP or AS. In the Insufficient ASP Resources case, the
SG is indicating to an "Inactive" ASP(s) in the AS that another ASP
is required in order to handle the load of the AS (Load-sharing
mode). For the Alternate ASP Active case, an ASP is informed when an
alternate ASP transitions to the ASP-Active state in Over-ride mode.
The format and description of the optional Interface Identifiers and
Info String parameters is the same as for the ASP Active message (See
Section 3.3.2.3.).
3.3.3.3 TEI Status Messages (Request, Confirm and Indication)
The TEI Status messages are exchanged between IUA layer peers to
request, confirm and indicate the status of a particular TEI.
The TEI Status messages contain the common message header followed by
IUA message header. The TEI Status Request message does not contain
any additional parameters.
In the integrated ISDN Layer 2/3 model (e.g., in traditional ISDN
switches), it is assumed that the Layer Management for the Q.921
Layer and the Q.931 layer are co-located. When backhauling ISDN,
this assumption is not necessarily valid. The TEI status messages
allow the two Layer Management entities to communicate the status of
the TEI. In addition, knowing that a TEI is in service allows the
ASP to request the SG to establish the datalink to the terminal (via
the IUA Establish message) for signaling if the ASP wants to be in
control of data link establishment. Another use of the TEI status
procedure is where the Layer Management at the ASP can prepare for
send/receive signaling to/from a given TEI and confirm/verify the
establishment of a datalink to that TEI. For example, if a datalink
is established for a TEI that the ASP did not know was assigned, the
ASP can check to see whether it was assigned or whether there was an
error in the signaling message. Also, knowing that a TEI is out of
service, the ASP need not request the SG to establish a datalink to
that TEI.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
The TEI Status Indication, and Confirm messages contain the following
parameter:
STATUS
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x10) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The valid values for Status are shown in the following table.
Define Value Description
ASSIGNED 0x0 TEI is considered assigned by Q.921
UNASSIGNED 0x1 TEI is considered unassigned by Q.921
4.0 Procedures
The IUA layer needs to respond to various primitives it receives from
other layers as well as messages it receives from the peer IUA layer.
This section describes various procedures involved in response to
these events.
4.1 Procedures to support service in section 1.4.1
These procedures achieve the IUA layer's "Transport of Q.921/Q.931
boundary" service.
4.1.1 Q.921 or Q.931 primitives procedures
On receiving these primitives from the local layer, the IUA layer
will send the corresponding QPTM message (Data, Unit Data, Establish,
Release) to its peer. While doing so, the IUA layer needs to fill
various fields of the common and specific headers correctly. In
addition the message needs to be sent on the SCTP stream that
corresponds to the D channel (Interface Identifier).
4.1.2 QPTM message procedures
On receiving QPTM messages from a peer IUA layer, the IUA layer on an
SG or MGC needs to invoke the corresponding layer primitives (DL-
ESTABLISH, DL-DATA, DL-UNIT DATA, DL-RELEASE) to the local Q.921 or
Q.931 layer.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 20014.2 Procedures to support service in section 1.4.2
These procedures achieve the IUA layer's "Support for Communication
between Layer Managements" service.
4.2.1 Layer Management primitives procedures
On receiving these primitives from the local Layer Management, the
IUA layer will provide the appropriate response primitive across the
internal local Layer Management interface.
An M-SCTP ESTABLISH request from Layer Management will initiate the
establishment of an SCTP association. An M-SCTP ESTABLISH confirm
will be sent to Layer Management when the initiated association set-
up is complete. An M-SCTP ESTABLISH indication is sent to Layer
Management upon successful completion of an incoming SCTP association
set-up from a peer IUA node
An M-SCTP RELEASE request from Layer Management will initiate the
tear-down of an SCTP association. An M-SCTP RELEASE confirm will be
sent by Layer Management when the association teardown is complete.
An M-SCTP RELEASE indication is sent to Layer Management upon
successful tear-down of an SCTP association initiated by a peer IUA.
M-SCTP STATUS request and indication support a Layer Management query
of the local status of a particular SCTP association.
M-NOTIFY indication and M-ERROR indication indicate to Layer
Management the notification or error information contained in a
received IUA Notify or Error message respectively. These indications
can also be generated based on local IUA events.
M-ASP STATUS request/indication and M-AS-STATUS request/indication
support a Layer Management query of the local status of a particular
ASP or AS. No IUA peer protocol is invoked.
M-ASP-UP request, M-ASP-DOWN request, M-ASP-INACTIVE request and M-
ASP-ACTIVE request allow Layer Management at an ASP to initiate state
changes. These requests result in outgoing IUA ASP UP, ASP DOWN, ASP
INACTIVE and ASP ACTIVE messages.
M-ASP-UP confirmation, M-ASP-DOWN confirmation, M-ASP-INACTIVE
confirmation and M-ASP-ACTIVE confirmation indicate to Layer
Management that the previous request has been confirmed.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
Upon receipt of a M-TEI Status primitive from Layer Management, the
IUA will send the corresponding MGMT message (TEI Status) to its
peer. While doing so, the IUA layer needs to fill various fields of
the common and specific headers correctly.
All MGMT messages are sent on a sequenced stream to ensure ordering.
SCTP stream '0' SHOULD be used.
4.2.2 Receipt of IUA Peer Management messages
Upon receipt of IUA Management messages, the IUA layer MUST invoke
the corresponding Layer Management primitive indications (e.g., M-AS
Status ind., M-ASP Status ind., M-ERROR ind., M-TEI STATUS...) to the
local layer management.
M-NOTIFY indication and M-ERROR indication indicate to Layer
Management the notification or error information contained in a
received IUA Notify or Error message. These indications can also be
generated based on local IUA events.
All MGMT messages are sent on a sequenced stream to ensure ordering.
SCTP stream '0' SHOULD be used.
4.3 Procedures to support service in section 1.4.3
These procedures achieve the IUA layer's "Support for management of
active associations between SG and MGC" service.
4.3.1 AS and ASP State Maintenance
The IUA layer on the SG needs to maintain the states of each ASP as
well as the state of the AS.
4.3.1.1 ASP States
The state of the each ASP, in each AS that it is configured, is
maintained in the IUA layer on the SG. The state of an ASP changes
due to the following type of events:
* Reception of messages from peer IUA layer at that ASP
* Reception of some messages from the peer IUA layer at other
ASPs in the AS
* Reception of indications from SCTP layer
The ASP state transition diagram is shown in Figure 7. The possible
states of an ASP are the following:
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
ASP-DOWN: Application Server Process is unavailable and/or the
related SCTP association is down. Initially, all ASPs will be in
this state. An ASP in this state SHOULD NOT be sent any IUA messages.
ASP-INACTIVE: The remote IUA peer at the ASP is available (and the
related SCTP association is up) but application traffic is stopped.
In this state the ASP can be sent any non-QPTM IUA messages (except
for TEI Status messages).
ASP-ACTIVE: The remote IUA peer at the ASP is available and
application traffic is active.
Figure 7 ASP State Transition Diagram
+-------------+
+----------------------| |
| Alternate +-------| ASP-ACTIVE |
| ASP | +-------------+
| Takeover | ^ |
| | ASP | | ASP
| | Active | | Inactive
| | | v
| | +-------------+
| | | |
| +------>| ASP-INACT |
| +-------------+
| ^ |
ASP Down/ | ASP | | ASP Down /
SCTP CDI | Up | | SCTP CDI
| | v
| +-------------+
+--------------------->| |
| ASP-DOWN |
+-------------+
SCTP CDI: The local SCTP layer's Communication Down Indication to
the Upper Layer Protocol (IUA) on an SG. The local SCTP will send
this indication when it detects the loss of connectivity to the ASP's
peer SCTP layer. SCTP CDI is understood as either a SHUTDOWN
COMPLETE notification and COMMUNICATION LOST notification from the
SCTP.
4.3.1.2 AS States
The state of the AS is maintained in the IUA layer on the SG.
The state of an AS changes due to events. These events include the
following:
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
* ASP state transitions
* Recovery timer triggers
The possible states of an AS are the following:
AS-DOWN: The Application Server is unavailable. This state implies
that all related ASPs are in the ASP-DOWN state for this AS.
Initially the AS will be in this state.
AS-INACTIVE: The Application Server is available but no application
traffic is active (i.e., one or more related ASPs are in the ASP-
INACTIVE state, but none in the ASP-ACTIVE state). The recovery
timer T(r) is not running or has expired.
AS-ACTIVE: The Application Server is available and application
traffic is active. This state implies that at least one ASP is in
the ASP-ACTIVE state.
AS-PENDING: An active ASP has transitioned from active to inactive or
down and it was the last remaining active ASP in the AS. A recovery
timer T(r) will be started and all incoming SCN messages will be
queued by the SG. If an ASP becomes active before T(r) expires, the
AS will move to AS-ACTIVE state and all the queued messages will be
sent to the active ASP.
If T(r) expires before an ASP becomes active, the SG stops queuing
messages and discards all previously queued messages. The AS will
move to AS-INACTIVE if at least one ASP is in ASP-INACTIVE state,
otherwise it will move to AS-DOWN state.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
Once the SCTP association is established and assuming that the local
IUA-User is ready, the local ASP IUA Application Server Process
Maintenance (ASPM) function will initiate the ASPM procedures, using
the ASP Up/-Down/-Active/-Inactive messages to convey the ASP state
to the SG - see Section 4.3.3.
The Layer Management and the IUA layer on SG can communicate the
status of the application server using the M-AS STATUS primitives.
The Layer Management and the IUA layer on both the SG and ASP can
communicate the status of an SCTP association using the M-SCTP STATUS
primitives.
If the Layer Management on SG or ASP wants to bring down an SCTP
association for management reasons, they would send M-SCTP RELEASE
request primitive to the local IUA layer. The IUA layer would
release the SCTP association and upon receiving the SCTP
Communication Down indication from the underlying SCTP layer, it
would inform the local Layer Management using M-SCTP RELEASE confirm
primitive.
If the IUA layer receives an SCTP-Communication Down indication from
the underlying SCTP layer, it will inform the Layer Management by
invoking the M-SCTP RELEASE indication primitive. The state of the
ASP will be moved to "Down" at both the SG and ASP.
At an ASP, the Layer Management MAY try to reestablish the SCTP
association using M-SCTP ESTABLISH request primitive.
4.3.3 ASPM procedures for peer-to-peer messages
All ASPM messages are sent on a sequenced stream to ensure ordering.
SCTP stream '0' SHOULD be used.
4.3.3.1 ASP Up
After an ASP has successfully established an SCTP association to an
SG, the SG waits for the ASP to send an ASP Up message, indicating
that the ASP IUA peer is available. The ASP is always the initiator
of the ASP Up exchange.
When an ASP Up message is received at an SG and internally the remote
ASP is not considered locked-out for local management reasons, the SG
marks the remote ASP as "Inactive". The SG responds with an ASP Up
Ack message in acknowledgement. The SG sends an ASP-Up Ack message
in response to a received ASP Up message even if the ASP is already
marked as "Inactive" at the SG.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
If for any local reason the SG cannot respond with an ASP Up, the SG
responds to a ASP Up with a with an ASP-Down Ack message with Reason
"Management Blocking".
At the ASP, the ASP Up Ack message received from the SG is not
acknowledged by the ASP. If the ASP does not receive a response from
the SG, or an ASP Down Ack is received, the ASP MAY resend ASP Up
messages every 2 seconds until it receives a ASP Up Ack message from
the SG. The ASP MAY decide to reduce the frequency (say to every 5
seconds) if an ASP Up Ack is not received after a few tries.
The ASP MUST wait for the ASP Up Ack message from the SG before
sending any ASP traffic control messages (ASPAC or ASPIA) or Data
messages or it will risk message loss. If the SG receives QPTM, ASP
Active or ASP Inactive messages before an ASP Up is received, the SG
SHOULD discard these messages.
4.3.3.2 ASP Down
The ASP will send an ASP Down to an SG when the ASP is to be removed
from the list of ASPs in all Application Servers that it is a member
and no longer receive any IUA traffic or management messages.
Whether the ASP is permanently removed from an AS is a function of
configuration management.
The SG marks the ASP as "Down" and returns an ASP Down Ack message to
the ASP if one of the following events occur:
- to acknowledge an ASP Down message from an ASP,
- to reply to ASPM messages from an ASP which is locked out for
management reasons.
The SG sends an ASP Down Ack message in response to a received ASP
Down message from the ASP even if the ASP is already marked as "Down"
at the SG.
If the ASP does not receive a response from the SG, the ASP MAY send
ASP Down messages every 2 seconds until it receives an ASP Down Ack
message from the SG or the SCTP association goes down. The ASP MAY
decide to reduce the frequency (say to every 5 seconds) if an ASP
Down Ack is not received after a few tries.
4.3.3.3 IUA Version Control
If a ASP Up message with an unsupported version is received, the
receiving end responds with an Error message, indicating the version
the receiving node supports and notifies Layer Management.
Morneault, et al. Standards Track [Page 52]

RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
This is useful when protocol version upgrades are being performed in
a network. A node upgraded to a newer version SHOULD support the
older versions used on other nodes it is communicating with. Because
ASPs initiate the ASP Up procedure it is assumed that the Error
message would normally come from the SG.
4.3.3.4 ASP Active
Any time after the ASP has received a ASP Up Ack from the SG, the ASP
sends an ASP-Active (ASPAC) to the SG indicating that the ASP is
ready to start processing traffic. In the case where an ASP is
configured/registered to process the traffic for more than one
Application Server across an SCTP association, the ASPAC contains one
or more Interface Identifiers to indicate for which Application
Servers the ASPAC applies.
When an ASP Active (ASPAC) message is received, the SG responds to
the ASP with a ASPAC Ack message acknowledging that the ASPAC was
received and starts sending traffic for the associated Application
Server(s) to that ASP.
The ASP MUST wait for the ASP-Active Ack message from the SG before
sending any Data messages or it will risk message loss. If the SG
receives QPTM messages before an ASP Active is received, the SG
SHOULD discard these messages.
There are two modes of Application Server traffic handling in the SG
IUA - Over-ride and Load-sharing. The Type parameter in the ASPAC
message indicates the mode used in a particular Application Server.
If the SG determines that the mode indicates in an ASPAC is
incompatible with the traffic handling mode currently used in the AS,
the SG responds with an Error message indicating Unsupported Traffic
Handling Mode.
In the case of an Over-ride mode AS, reception of an ASPAC message at
an SG causes the redirection of all traffic for the AS to the ASP
that sent the ASPAC. The SG responds to the ASPAC with an ASP-Active
Ack message to the ASP. Any previously active ASP in the AS is now
considered Inactive and will no longer receive traffic from the SG
within the AS. The SG sends a Notify (Alternate ASP-Active) to the
previously active ASP in the AS, after stopping all traffic to that
ASP.
In the case of a load-share mode AS, reception of an ASPAC message at
an SG causes the direction of traffic to the ASP sending the ASPAC,
in addition to all the other ASPs that are currently active in the
AS. The algorithm at the SG for load-sharing traffic within an AS to
all the active ASPs is implementation dependent. The algorithm
Morneault, et al. Standards Track [Page 53]

RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
could, for example be round-robin or based on information in the Data
message, such as Interface Identifier, depending on the requirements
of the application and the call state handling assumptions of the
collection of ASPs in the AS. The SG responds to the ASPAC with a
ASP-Active Ack message to the ASP.
4.3.3.5 ASP Inactive
When an ASP wishes to withdraw from receiving traffic within an AS,
the ASP sends an ASP Inactive (ASPIA) to the SG. In the case where
an ASP is configured/registered to process the traffic for more than
one Application Server across an SCTP association, the ASPIA contains
one or more Interface Identifiers to indicate for which Application
Servers the ASPIA applies.
There are two modes of Application Server traffic handling in the SG
IUA when withdrawing an ASP from service - Over-ride and Load-
sharing. The Type parameter in the ASPIA message indicates the mode
used in a particular Application Server. If the SG determines that
the mode indicates in an ASPAC is incompatible with the traffic
handling mode currently used in the AS, the SG responds with an Error
message indicating Unsupported Traffic Handling Mode.
In the case of an Over-ride mode AS, where normally another ASP has
already taken over the traffic within the AS with an Over-ride ASPAC,
the ASP which sends the ASPIA is already considered by the SG to be
"Inactive". An ASPIA Ack message is sent to the ASP, after ensuring
that all traffic is stopped to the ASP.
In the case of a Load-share mode AS, the SG moves the ASP to the
"Inactive" state and the AS traffic is re-allocated across the
remaining "active" ASPs per the load-sharing algorithm currently used
within the AS. An ASPIA Ack message is sent to the ASP after all
traffic is halted to the ASP. A NTFY (Insufficient ASPs) MAY be sent
to all inactive ASPs, if required.
If no other ASPs are Active in the Application Server, the SG sends a
NTFY (AS-Pending) to all inactive ASPs of the AS and either discards
all incoming messages for the AS or starts buffering the incoming
messages for T(r)seconds, after which messages will be discarded.
T(r) is configurable by the network operator. If the SG receives an
ASPAC from an ASP in the AS before expiry of T(r), the buffered
traffic is directed to the ASP and the timer is cancelled. If T(r)
expires, the AS is moved to the "Inactive" state.
Morneault, et al. Standards Track [Page 54]

RFC 3057 ISDN Q.921-User Adaptation Layer February 20014.3.3.6 Notify
A Notify message reflecting a change in the AS state is sent to all
ASPs in the AS, except those in the "Down" state, with appropriate
Status Identification.
In the case where a Notify (AS-Pending) message is sent by an SG that
now has no ASPs active to service the traffic, or a NTFY
(Insufficient ASPs) is sent in the Load-share mode, the Notify does
not explicitly force the ASP(s) receiving the message to become
active. The ASPs remain in control of what (and when) action is
taken.
4.3.3.7 Heartbeat
The optional Heartbeat procedures MAY be used when operating over
transport layers that do not have their own heartbeat mechanism for
detecting loss of the transport association (i.e., other than the
SCTP).
After receiving an ASP Up Ack message from the SG in response to an
ASP Up message, the ASP MAY optionally send Beat messages
periodically, subject to a provisionable timer T(beat). The SG IUA,
upon receiving a BEAT message from the ASP, responds with a BEAT ACK
message. If no BEAT message (or any other IUA message) is received
from the SG within the timer 2*T(beat), the SG will consider the
remote IUA as "Down". The SG will also send an ASP Down Ack message
to the ASP.
At the ASP, if no BEAT ACK message (or any other IUA message) is
received from the SG within 2*T(beat), the SG is considered
unavailable. Transmission of BEAT messages is stopped and ASP Up
procedures are used to re-establish communication with the SG IUA
peer.
The BEAT message MAY optionally contain an opaque Heartbeat Data
parameter that MUST be echoed back unchanged in the related Beat Ack
message. The ASP upon examining the contents of the returned BEAT
Ack message MAY choose to consider the remote ASP as unavailable.
The contents/format of the Heartbeat Data parameter is
implementation-dependent and only of local interest to the original
sender. The contents MAY be used, for example, to support a
Heartbeat sequence algorithm (to detect missing Heartbeats), and/or a
timestamp mechanism (to evaluate delays).
Note: Heartbeat related events are not shown in Figure 4 "ASP state
transition diagram".
Morneault, et al. Standards Track [Page 55]

RFC 3057 ISDN Q.921-User Adaptation Layer February 20015.0 Examples5.1 Establishment of Association and Traffic between SGs and ASPs5.1.1 Single ASP in an Application Server (1+0 sparing)
This scenario shows the example IUA message flows for the
establishment of traffic between an SG and an ASP, where only one ASP
is configured within an AS (no backup). It is assumed that the SCTP
association is already set-up.
SG ASP1
|
|<---------ASP Up----------|
|--------ASP Up Ack------->|
| |
|<-------ASP Active--------|
|------ASP Active Ack----->|
| |
5.1.2 Two ASPs in Application Server (1+1 sparing)
This scenario shows the example IUA message flows for the
establishment of traffic between an SG and two ASPs in the same
Application Server, where ASP1 is configured to be Active and ASP2 a
standby in the event of communication failure or the withdrawal from
service of ASP1. ASP2 MAY act as a hot, warm, or cold standby
depending on the extent to which ASP1 and ASP2 share call state or
can communicate call state under failure/withdrawal events. The
example message flow is the same whether the ASP-Active messages are
Over-ride or Load-share mode although typically this example would
use an Over-ride mode.
SG ASP1 ASP2
| | |
|<--------ASP Up----------| |
|-------ASP Up Ack------->| |
| | |
|<-----------------------------ASP Up----------------|
|----------------------------ASP Up Ack------------->|
| | |
| | |
|<-------ASP Active-------| |
|-----ASP Active Ack----->| |
| | |
Morneault, et al. Standards Track [Page 56]

RFC 3057 ISDN Q.921-User Adaptation Layer February 20015.1.3 Two ASPs in an Application Server (1+1 sparing, load-sharing case)
This scenario shows a similar case to Section 5.1.2 but where the two
ASPs are brought to active and load-share the traffic load. In this
case, one ASP is sufficient to handle the total traffic load.
SG ASP1 ASP2
| | |
|<---------ASP Up---------| |
|--------ASP Up Ack------>| |
| | |
|<------------------------------ASP Up---------------|
|-----------------------------ASP Up Ack------------>|
| | |
| | |
|<--ASP Active (Ldshr)----| |
|----ASP Active Ack------>| |
| | |
|<----------------------------ASP Active (Ldshr)-----|
|-----------------------------ASP Active Ack-------->|
| | |
5.1.4 Three ASPs in an Application Server (n+k sparing, load-sharing case)
This scenario shows the example IUA message flows for the
establishment of traffic between an SG and three ASPs in the same
Application Server, where two of the ASPs are brought to active and
share the load. In this case, a minimum of two ASPs are required to
handle the total traffic load (2+1 sparing).
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
- Send the QPTM message to the remote IUA peer in the SG, over
the SCTP association
When the IUA layer on the SG has a QPTM message to send to the ASP,
it will do the following:
- Determine the AS for the Interface Identifier
- Determine the Active ASP (SCTP association) within the AS
- Determine the correct stream in the SCTP association based on
the D channel
- Fill in the QPTM message, fill in IUA Message Header, fill in
Common Header
- Send the QPTM message to the remote IUA peer in the ASP, over
the SCTP association
An example of the message flows for establishing a data link on a
signaling channel, passing PDUs and releasing a data link on a
signaling channel is shown below. An active association between MGC
and SG is established (Section 5.1) prior to the following message
flows.
SG ASP
<----------- Establish Request
Establish Confirm ---------->
<----------- Data Request
Data Indication ----------->
<----------- Data Request
Data Indication ----------->
<----------- Data Request
<----------- Data Request
Data Indication ----------->
<----------- Release Request (RELEASE_MGMT)
Release Confirm ---------->
An example of the message flows for a failed attempt to establish a
data link on the signaling channel is shown below. In this case, the
gateway has a problem with its physical connection (e.g., Red Alarm),
so it cannot establish a data link on the signaling channel.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
SG ASP
<----------- Establish Request (ESTABLISH_START)
Release Indication ---------->
(RELEASE_PHYS)
5.4 Layer Management Communication Examples
An example of the message flows for communication between Layer
Management modules between SG and ASP is shown below. An active
association between ASP and SG is established (Section 5.1) prior to
the following message flows.
SG ASP
<----------- Data Request
Error Indication ---------->
(INVALID_TEI)
<----------- TEI Status Request
TEI Status Confirm ---------->
(Unassigned)
6.0 Security
IUA is designed to carry signaling messages for telephony services.
As such, IUA MUST involve the security needs of several parties the
end users of the services; the network providers and the applications
involved. Additional requirements MAY come from local regulation.
While having some overlapping security needs, any security solution
SHOULD fulfill all of the different parties' needs.
6.1 Threats
There is no quick fix, one-size-fits-all solution for security. As a
transport protocol, IUA has the following security objectives:
* Availability of reliable and timely user data transport.
* Integrity of user data transport.
* Confidentiality of user data.
IUA runs on top of SCTP. SCTP [3] provides certain transport related
security features, such as
* Blind Denial of Service Attacks
* Flooding
* Masquerade
* Improper Monopolization of Services
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
When IUA is running in professionally managed corporate or service
provider network, it is reasonable to expect that this network
includes an appropriate security policy framework. The "Site
Security Handbook" [5] SHOULD be consulted for guidance.
When the network in which IUA runs in involves more than one party,
it MAY NOT be reasonable to expect that all parties have implemented
security in a sufficient manner. In such a case, it is recommended
that IPSEC is used to ensure confidentiality of user payload.
Consult [6] for more information on configuring IPSEC services.
6.2 Protecting Confidentiality
Particularly for mobile users, the requirement for confidentiality
MAY include the masking of IP addresses and ports. In this case
application level encryption is not sufficient; IPSEC ESP SHOULD be
used instead. Regardless of which level performs the encryption, the
IPSEC ISAKMP service SHOULD be used for key management.
7.0 IANA Considerations7.1 SCTP Payload Protocol Identifier
A request will be made to IANA to assign an IUA value for the Payload
Protocol Identifier in SCTP Payload Data chunk. The following SCTP
Payload Protocol Identifier will be registered:
IUA "1"
The SCTP Payload Protocol Identifier is included in each SCTP Data
chunk, to indicate which protocol the SCTP is carrying. This Payload
Protocol Identifier is not directly used by SCTP but MAY be used by
certain network entities to identify the type of information being
carried in a Data chunk.
The User Adaptation peer MAY use the Payload Protocol Identifier as a
way of determining additional information about the data being
presented to it by SCTP.
7.2 IUA Protocol Extensions
This protocol may also be extended through IANA in three ways:
-- through definition of additional message classes,
-- through definition of additional message types, and
-- through definition of additional message parameters.
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RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
The definition and use of new message classes, types and parameters
is an integral part of SIGTRAN adaptation layers. Thus, these
extensions are assigned by IANA through an IETF Consensus action as
defined in [RFC2434].
The proposed extension must in no way adversely affect the general
working of the protocol.
7.2.1 IETF Defined Message Classes
The documentation for a new message class MUST include the following
information:
(a) A long and short name for the message class.
(b) A detailed description of the purpose of the message class.
7.2.2 IETF Defined Message Types
Documentation of the message type MUST contain the following
information:
(a) A long and short name for the new message type.
(b) A detailed description of the structure of the message.
(c) A detailed definition and description of intended use of each
field within the message.
ti3 (d) A detailed procedural description of the use of the new
message type within the operation of the protocol.
(e) A detailed description of error conditions when receiving this
message type.
When an implementation receives a message type which it does not
support, it MUST respond with an Error (ERR) message with an Error
Code of Unsupported Message Type.
7.2.3 IETF-defined TLV Parameter Extension
Documentation of the message parameter MUST contain the following
information:
(a) Name of the parameter type.
(b) Detailed description of the structure of the parameter field.
This structure MUST conform to the general type-length-value
format described in Section 3.1.5.
(c) Detailed definition of each component of the parameter value.
(d) Detailed description of the intended use of this parameter type,
and an indication of whether and under what circumstances
multiple instances of this parameter type may be found within the
same message type.
Morneault, et al. Standards Track [Page 63]

RFC 3057 ISDN Q.921-User Adaptation Layer February 200110. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
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followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Morneault, et al. Standards Track [Page 66]